1) Field of the Invention
The invention relates to a process for preparing crosslinkable binders and to the use of the vinyl ester-olefin copolymers obtainable therewith and stabilized with protective colloid as binders, in particular as binders for elastic coatings.
2) Background Art
Aqueous synthetic-resin dispersions and corresponding powders have for many years now been indispensable additives for application in the construction sector, especially for plasters, mortars, reinforcing compounds, self-leveling compounds, tile adhesives, paints, and integrated heat insulation systems. For high-grade plasters and paints, in particular, there is ever-increasing use of binders which give these products properties such as improved mechanical qualities, better weathering resistance, and a reduced soiling tendency.
In the exterior sector in particular, good mechanical properties are necessary in order to bridge the structural cracks that appear in the coating over the course of time. In order to achieve this, it is common to use binders whose base resins have very low glass transition temperatures (from -20.degree. C. to -50.degree. C.). Because of these extremely low Tgs, the binders possess very good low-temperature elasticity. On the other hand, the surface of these coatings as a result is naturally very soft and tacky and exhibits an increased tendency to attract dirt, which is a great disadvantage in particular in regions with adverse weather conditions. For this end use, therefore, the prior art recommends post-reactive systems, which crosslink by initiation with sunlight (UV radiation), cure superficially, and so become resistant to soiling.
EP-A 499835 (U.S. Pat. No. 5,314,936) and DE-A 4318083 disclose binder compositions for coating materials, comprising carbonyl- or amido-functional, emulsifier-stabilized copolymers and aromatic ketones for their UV crosslinking. EP-A 441221 relates to UV-crosslinkable aqueous coating materials based on an emulsifier-stabilized acrylate and/or vinyl ester polymer dispersion and aromatic ketones for preventing efflorescence phenomena on mineral substrates. Non-tacky, non-soiling, flexible coatings based on an (meth)acrylate polymer dispersion and water-soluble zinc-amine complexes are known from EP-A 324416 (U.S. Pat No. 4,980,411).
EP-A 565093 describes graft copolymers formed from siloxane and acrylate monomer, which exhibit good extension and a low soiling tendency owing to the silicone component. The crosslinking of the coating material is accelerated by the condensation catalyst fraction. EP-A 421787 discloses coating materials for producing elastic coatings, these materials consisting of two incompatible polymers of which one has N-methylolacrylamide units. Nothing is said about the tendency toward soiling prior to UV exposure.
EP-A 522789 discloses aqueous dispersions for preparing elastic coatings, obtained by first polymerizing a relatively soft polymer and then polymerizing a relatively hard polymer. The resulting core-shell polymer dispersions are equipped with photosensitizers for UV crosslinking, and the soiling tendency of the resultant coatings is investigated before and after UV irradiation.
EP-A 602763 relates to UV-crosslinkable coating materials based on 2-stage polymers, the first stage comprising polymers with polyethylenically unsaturated monomers and the second stage constituting an acid-functional polymer which can be crosslinked after neutralization with epoxy.
A disadvantage of all of the systems referred to is that, despite the fact that following UV irradiation, i.e., in sunlight, they lead to relatively soiling-resistant coatings, without UV irradiation they are extremely susceptible to soiling. Especially in the case of facing coatings in shady areas, or on weathering, the soft coatings remain uncrosslinked and so susceptible to soiling. In rainy weather prior to or following the application of said systems, moreover, there is also a risk of the photoinitiator being washed out, which results in a great reduction in the UV sensitivity and, ultimately, in unsatisfactory crosslinking and soiling resistance of the coatings.
It is therefore an object of the present invention to provide binders, especially for elastic coatings, which are resistant to soiling on their surface both in the dark and following UV radiation but which nevertheless have good low-temperature elasticity.